Add shading to color of sidebar, for contrast against main pane

[brainchild0]

Testing under versions 25 and 26, the sidebar and the main pane have identical background colors, in some cases.

Such lack of contrast leads to effects at the physical boundary that may be perceived as unsettling to the viewer. The conceptual relation of the sidebar occluding a region of the main pane is not clearly expressed by the choice of graphical representations

Suggested is some kind of contrasted coloring, whether involving a slight shading of the background, or some other scheme.

As an example, the extension Custom Menu features a side bar whose background color is by default based on the site theme color, but with added shading.

Some applications appear to have resolved the problem similarly to as suggested, but others have not.

A shaded variant of either the site theme color or the general background color may be a more suitable choice for the background color of the sidebar, compared to the identical color of the general background.

[brainchild0]

I am not reproducing the observations under Nextcloud 27.

I am concluding that the issue was addressed by recent enhancements.